This invention pertains to the fabrication of semi-conductor devices, and particularly to dry plasma etching processes used in such fabrication.
Semi-conductor devices, such as semi-conductor integrated circuits, typically comprise a substrate, or base, which has deposited thereon one or more film layers. Some such film layers have a dopant selectively diffused or otherwise loaded therein in accordance with a pattern indicative of the junctions to be formed in the integrated circuit. The pattern may correspond, for example, to portions of the film not temporarily covered by a photoresist or mask which shields the portions not to be doped. Before diffusing or loading the dopant into the film, however, select portions of the film layer must first be removed, or etched, in accordance with the desired pattern.
One known technique for etching a film is plasma etching. Plasma etching is the selective removal of material by reaction with chemically active gases created by a radio frequency (RF) power-induced glow discharge environment. In plasma etching, the RF voltage applied across the gas normally causes the gases to disassociate and to form various free radicals which chemically interact to etch away the select areas of the film for subsequent doping.
Apparatus used for plasma etching is of two basic types. In the first, one or more electrodes or an inductance coil is external to a vacuum chamber into which a semi-conductor workpiece is inserted. In the second type of apparatus, the workpiece is placed on a cathode which, along with an anode, is internally contained in the vacuum chamber.
While plasma etching is generally thought to be a cleaner, more economical, and more desirable technique than the wet chemical etching techniques heretofore known, various factors (such as the composition of the film-to-be-etched and any neighboring films, as well as the composition of the gas employed in the process) determine whether the particular plasma etching process employed overcomes various potential problems associated with the process. Such problems include, for example, the fact that in some processes the etching rate of the exposed portions of the film-to-be-etched is not significantly greater than that of neighboring or underlying films which are not to be etched. Likewise, in some instances, the gas plasma has a deleterious effect on the photoresist which serves to shield the portions of the film which are not to be etched.
Another important consideration in the plasma etching process is the degree of accuracy or sharpness with which the plasma etches only the selected portions of the film. The plasma should not, while etching an exposed portion of the film, undercut into a neighboring portion of the same film which is not to be etched.
Various semi-conductor devices currently available comprise a polycrystalline silicon (polysilicon) film deposited on a substrate. In many cases the substrate has an oxidized silicon coating onto which the polysilicon is deposited. However, the particular gases presently employed to etch a film, such as a polysilicon film, do not favorably solve problems such as those mentioned above.